The exemplary embodiments described herein relate to methods for producing polyolefin polymers.
Polyolefin polymers may be produced using gas phase polymerization processes. In a typical gas-phase fluidized bed polymerization process, a gaseous stream containing one or more monomers is continuously passed through the fluidized bed under reactive conditions in the presence of a catalyst. The gaseous stream is withdrawn from the fluidized bed and recycled back into the reactor. Generally, the recycled gas stream is heated in the reactor by the heat of polymerization. This heat may be removed in another part of the cycle (e.g., by a cooling system external to the reactor such as a heat exchanger), so as to maintain the temperature of the resin and gaseous stream inside the reactor below the polymer melting point or the catalyst deactivation temperature.
Heat removal may also help prevent excessive stickiness of polymer particles that may result in agglomeration. Particle agglomerations may lead to the formation of chunks or sheets of polymer that cannot be removed from the reactor as product. Further, such chunks or sheets may fall onto the reactor distributor plate which may impair fluidization of the bed and may lead to a discontinuity event. Additionally, since the polymerization reaction is exothermic, the amount of polymer produced in a fluidized bed polymerization process is related to the amount of heat that can be withdrawn from the reaction zone.
For a time, it was thought that the temperature of the gaseous stream external to the reactor, otherwise known as the recycle stream temperature, could not be decreased below the dew point of the recycle stream without causing problems such as polymer agglomeration or plugging of the reactor system. The dew point of the recycle stream is the temperature at which liquid condensate first begins to form in the gaseous recycle stream, which can be calculated knowing the gas composition and is thermodynamically defined using an equation of state. However, it was found that in some instances a recycle stream may be cooled to a temperature below the dew point in a fluidized bed polymerization process resulting in condensing a portion of the recycle gas stream outside of the reactor. The resulting stream containing entrained liquid can then be returned to the reactor without causing agglomeration or plugging phenomena. The process of purposefully condensing a portion of the recycle stream is known in the industry as “condensed mode” operation. When a recycle stream temperature is lowered to a point below its dew point in condensed mode operation, an increase in polymer production may be possible.
Cooling of the recycle stream to a temperature below the gas dew point temperature produces a two-phase gas/liquid mixture that may have entrained solids contained in both phases. The liquid phase of this two-phase gas/liquid mixture in condensed mode operation is generally entrained in the gas phase of the mixture. Vaporization of the liquid occurs only when heat is added or pressure is reduced. Generally, the vaporization occurs when the two-phase mixture enters the fluidized bed, with the resin providing the required heat of vaporization. The vaporization thus provides an additional means of extracting heat of reaction from the fluidized bed.
The cooling capacity of the recycle gas may be increased further while at a given reaction temperature and a given temperature of the cooling heat transfer medium. This can be performed by adding non-polymerizing, non-reactive materials to the reactor, which are condensable at the temperatures encountered in the process heat exchanger. Such materials are collectively known as induced condensing agents (ICA). Increasing concentrations of an ICA in the reactor cause corresponding increases in the dew point temperature of the reactor gas, which promotes higher levels of condensing for higher (heat transfer limited) production rates from the reactor. However, increasing the ICA concentration in the reactor changes the melt index of the produced polyolefin, which is often undesirable because the melt index of the polyolefin is one of the crucial polymer properties to customers that later produce articles like disposable bags and medical devices therefrom. Even small deviations in melt index outside a specified range can affect the processability of the polyolefin when producing downstream articles.